Linking microbial carbon pump capacity and efficacy to soil organic carbon storage and stability under heavy metal pollution

Heavy metal pollution can lead to a great loss of soil organic carbon (SOC). However, the microbial mechanisms that link heavy metal pollution to SOC remain poorly understood. Here, we investigated five apple-orchard soils at different distances from a Pb−Zn smelter. After assessing the heavy metal pollution level based on Grade II of the national soil environmental quality standard (China), we found SOC stocks and microbial carbon pump (MCP) capacity (i.e., microbial residue carbon) under medium and heavy pollution levels were significantly lower than those under safe, cordon and light pollution levels. The structural equation model showed causality in the SOC variations linked to pollution level through MCP capacity, which could contribute 77.8% of the variance in SOC storage. This verified MCP capacity can serve as a key parameter for evaluation of SOC storage under heavy metal pollution. Soil MCP efficacy, i.e., the proportion of microbial residue carbon to SOC, also decreased under medium and heavy pollution. This suggested that, with a heavier pollution level, there was a higher rate of reduction of microbial residue carbon in soil than the rate of reduction of SOC. As MCP efficacy can be a useful assessment of SOC stability, the significantly positive relationship between MCP efficacy and clay content in correlation analysis implied that lower MCP efficacy was correlated with SOC stability under the heavier pollution level. Our study provides valuable insights to identify the mechanisms of microbially mediated C transformation processes that are influenced by heavy metal pollution in agroecosystems.